1. Field of the invention
This invention relates to a semiconductor laser device which oscillates laser light with an oscillation wavelength in the visible region, especially, a high quality semiconductor laser device which can be readily produced by molecular beam epitaxy.
2. Description of the prior art
In recent years, single thin crystal film growth techniques such as molecular beam epitaxy (MBE) and metal organic-chemical vapor deposition (MO-CVD), have been rapidly advanced By these growth techniques, it is possible to obtain epitaxial growth layers of extreme thinness, on the order of 10 .ANG.. Due to the progress in these crystal growth techniques, it is possible to make laser devices based on device structures having very thin layers, which could not be easily manufactured by conventional liquid phase epitaxy. A typical example of these laser devices is the quantum well (QW) laser, in which the active layer
20 has a thickness of 100 .ANG. or less resulting in the formation of quantum levels therein, whereas the active layer of the conventional double-heterostructure (DH) laser has a thickness of several hundreds of angstroms or more. Thus, this QW laser is advantageous over the conventional DH laser in that the threshold current level is reduced, the temperature characteristics are excellent, and the transient characteristics are excellent. This has been reported by W. T. Tsang, Applied Physics Letters, vol. 39, No. 10, pp. 786 (1981); N. K. Dutta, Journal of Applied Physics, vol. 53, No. 11, pp. 7211 (1982); and H. Iwamura, T. Saku, T. Ishibashi, K. Otsuka, Y. Horikoshi, Electronics Letters, vol. 19, No. 5, pp. 180 (1983).
As mentioned above, by the use of single thin crystal film growth techniques such as MBE and MO-CVD, it is now possible to put high quality semiconductor lasers having a new multiple-layered structure into practical use.
A typical structure for conventional QW lasers is the AlGaAs laser with a graded-index separate-confinement heterostructure (GRIN-SCH). The AlAs mole fraction in the active region of these semiconductor lasers is shown in FIG. 5. The GaAs-GaAlAs lasers containing GaAs as a quantum well layer is produced by MBE as follows: The GRIN region 42 is grown by a decrease in the Al cell temperature from the outer surface of the cladding layer 41, which constitutes a double-heterostructure made by sandwiching the active layer between the cladding layers. Then, by closing the Al cell shutter, the GaAs quantum well layer 43 is grown. Thereafter, the Al cell shutter is opened again and the Al cell temperature is raised so as to grow the GRIN region 44 on the GaAs quantum well layer 43, followed by growing the cladding layer 45 on the GRIN region 44. Therefore, by regulating the Al cell temperature and the opening and closing of the shutter, the crystal growth can be achieved with the use of only one Al cell, which means that the MBE apparatus required is not complicated. However, when this structure is applied to a visible-light laser device having an AlGaAs quantum well layer therein, the AlAs mole fraction in the active region is as shown in FIG. 6, and two Al cells must be used. One of the Al cells is opened so as to give an AlAs mole fraction of 0.3, and the other Al cell controls the cladding layers 3 and 7 and the GRIN regions 4 and 6. This is why two Al cells are required. Moreover, bulk AlGaAs is used as a quantum well layer, which causes difficulties in maintaining the radiation efficiency at a high level.